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Investigation Regarding the Role of Chloride in Organic-Inorganic Halide Perovskites Obtained from Chloride Containing Precursors M. Ibrahim Dar, Neha Arora, Peng Gao, Shahzada Ahmad, Michael Grätzel, and Mohammad Khaja Nazeeruddin Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl503279x • Publication Date (Web): 13 Nov 2014 Downloaded from http://pubs.acs.org on November 17, 2014
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Investigation Regarding the Role of Chloride in Organic-Inorganic Halide Perovskites Obtained from Chloride Containing Precursors M. Ibrahim Dar,*a Neha Arora, a Peng Gao,a Shahzada Ahmad,b Michael Grätzel*a and Mohammad Khaja Nazeeruddin*a a
Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss
Federal Institute of Technology (EPFL), Station 6, 1015 Lausanne, Switzerland b
Abengoa Research, C/ Energía Solar nº 1, Campus Palmas Altas-41014, Sevilla, Spain
ABSTRACT: As the photovoltaic performance of a device is strongly influenced by the morphology of perovskite, achieving precise control over the crystal formation of organicinorganic halide perovskites synthesized in the ambience of chloride ions has garnered much attention. Although the resulting morphology dictates the performance of the device considerably, the understanding of the role of chloride ions has been scant. To unravel this mystery, we investigated three different organic-inorganic halide perovskite materials grown from the chloride-containing precursors under different but optimized conditions. Despite the presence of chloride ions in the reaction mixture, scanning transmission electron microscopyenergy dispersive spectroscopy (STEM-EDS) reveals that the CH3NH3PbI3 perovskites formed
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are chloride-free. Moreover bright field transmission electron microscopy indicates that chloride ions effect the growth of the CH3NH3PbI3.
KEYWORDS Perovskite, CH3NH3PbI3, Photovoltaic, Electron microscopy, STEM Organic-inorganic halide perovskites have recently gained prominence due to the development of high efficiency solar cells.1,2 Because of their structural and compositional diversity, these materials have been identified as candidates in a myriad of applications such as photovoltaics and luminescent displays.3,4,5,6 The properties of organic-inorganic halide perovskites are a function of dimensions, morphology, phase, and chemical composition which, together, eventually define the performance of the solar cell based on these materials.7,8,9 Therefore, gaining a rational control over the phase as well as the morphology of organic-inorganic halide perovskites has been pursued intensively.7,8 In controlling the growth of the perovskite layer, solution-based bottom-up approaches provide more flexibility than other methods employed for the fabrication of high-efficiency devices. By adopting ‘‘solution chemistry’’ for the synthesis of various organic-inorganic mainly two dimensional lead/tin-based halide perovskites, Mitzi and co-workers pioneered the two-step approach.5,6 Earlier, the fabrication of such devices employed a single-step deposition of the CH3NH3PbI3 layer10,11 which was succeeded by the sequential, two-step conversion method for the deposition of the CH3NH3PbI3 layer onto a mesoporous TiO2 film which provided a better control over the growth of perovskite structures.1,12,13 Two-step methodology involves the deposition of PbX2 (X= Cl, Br, I) onto a substrate and the resulting film is subsequently transformed into CH3NH3PbX3 by dipping into a solution of CH3NH3X3 whereas, single step methodology involves the deposition of mixture of PbX2 and CH3NH3X3 onto a substrate.14,11
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In addition to the development of fabrication methods for the preparation of perovskites, there has been an upsurge of interest in understanding their composition. This is primarily because composition dictates the property of the perovskite material significantly which, in turn, determines the performance of the resulting device.15 In principle, the desired properties of a hybrid organic-inorganic halide perovskite can be achieved by appropriately changing the organic cation, central metal ion, or the halide ion.7,9,15,16,17,18 In addition to other compositions, CH3NH3PbI3 and CH3NH3PbI3−xClx have largely been explored as the light-harnessing material in perovskite solar cells.1,2 One of the most remarkable properties associated with these perovskites is that the excitons in the solar cell can dissociate either in the bulk or at the donoracceptor interface, and owing to the long carrier lifetime and good carrier transport characteristics, charge extraction remains efficient.19,20 Although the presence of the chloride ion supposedly changes the properties of the resulting perovskites, i.e., CH3NH3PbI3−xClx, questions surround the presence and understanding the role of the chloride. To answer these questions, we have selected three perovskite systems synthesized by different approaches under optimized conditions. We have exploited electron microscopy to determine the content and distribution of the constituents, particularly the chloride ions in the perovskites. Using scanning transmission electron microscopy energy-dispersive spectroscopy (STEM-EDS), we conclude that the presence of the chloride ion as a substitutional dopant in the perovskite is not detectable; indeed it was found in the form of unconverted PbCl2. To the best of the authors’ knowledge, so far such an investigation has been scantly carried out on CH3NH3PbI3 materials. Such a technique, i.e., STEM-EDS exhibits several advantages over other techniques used for chemical analysis such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM)-EDS. Both XPS and SEM-EDS characterization techniques analyze the samples at bulk scale whereas
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STEM-EDS can bring out information locally, i.e., at sub nanometer (nm) level. In addition to chemical analysis, we also carried out extensive morphological and structural characterization using transmission electron microscopy (TEM) in bright field-TEM (BFTEM) and electron diffraction (SAED) mode. Our unprecedented study indicates that the chloride ions influence the growth of the CH3NH3PbI3 significantly without entering into its lattice. Based on the experimental conditions that were optimized for the synthesis of perovskite materials, four different samples were obtained as summarized in Table 1. The resulting perovskite structures were thoroughly examined for their morphology, size, composition, crystallinity, and dispersion by electron microscopy. Table1. Summary of the experimental conditions used for the synthesis of different perovskite structures.
Sample code
Precursors used
P1
PbI2, MAI:MACl (95:5) wt% Two-step in (8 mg in 1 mL of IPA)
4 min
110 °C for 30 min
P2
PbCl2 and MAI (8 mg in 1 Two-step mL of IPA)
4 min
110 °C for 30 min
P3
PbCl2: MAI= 1:3
------
110 °C for 30 min
P4
PbI2 and MAI (8 mg in 1 mL Two-step of IPA)
4 min
70 °C for 30 min
Approach
Single-step
Dipping time Annealing condition
*(MAI, MACl and IPA are the acronyms for methyl ammonium iodide, methyl ammonium chloride and isopropyl alcohol , respectively).
Morphological Characterization. To analyze the gross morphology of perovskite structures deposited on TiO2 photoanode, field emission scanning electron microscopy (FESEM) was used. The morphology obtained by SEM of the three different samples prepared in the presence of
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chloride ion is shown in Figure 1. The micrograph of P1 displays the formation of perovskite crystals which are polydisperse (Figure 1a). In addition to structures measuring ~100-300 nm in diameter, well–faceted, elongated perovskite structures (Figure 1b) with a high aspect ratio were also observed.
Figure 1. SEM analysis displaying the top view of perovskite structures; Low-magnification and high-magnification SEM micrographs of samples; P1 (a, b), P2 (c, d), and P3 (e, f). To understand the formation of such elongated structures with an average length of ~1000 nm (as evident from high-magnification cross sectional SEM, see S.I.), we synthesized CH3NH3PbI3 using pure MAI, i.e., no MACl was used (P4). Expectedly, we did not observe the formation of any elongated structures under similar conditions (see S.I.), suggesting that chloride ions from the MACl precursor might promote growth along a particular crystallographic direction. Nevertheless, prolonged dipping of PbI2 film into pure MAI solution leads to the evolution of
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large and faceted CH3NH3PbI3 structures (see S.I.). Formation of larger perovskite crystals could be explained by evoking the phenomenon of ripening. Low-magnification SEM micrograph of P2 (Figure 1c) displays the formation of highly dense, uniformly distributed perovskite structures. High-magnification SEM imaging (Figure 1d) discriminated between asymmetric and elongated perovskite structures. Although P1 and P2 were obtained using a two-step method involving chloride content, structures in P1 are well faceted and more elongated than in P2, which possibly explains the dominant role of the chloride ions present in the solution in orienting the growth of perovskite structures. In contrast to P1 and P2, the perovskite (P3) was deposited using a single-step approach, which evidently leads to inhomogeneous deposition of perovskite. Low-magnification SEM (Figure 1e) reveals the formation of interlinked structures, whereas high-magnification SEM (Figure 1f) ostensibly brings out that the mesoporous TiO2 substrate is partially covered with a perovskite overlayer. Such an observation is consistent with the literature where it has been documented that a slowly evaporating solvent such as DMSO reduces the surface coverage of perovskite.21 Employing a solvent like DMF was not considered as an alternative because PbCl2 shows lower solubility in it.22 From the comparative SEM analysis of P1, P2, and P3, it could be contended that the sequential technique provides superior control over the growth of perovskite and that crystal formation might occurs under near-equilibrium conditions, as the resulting CH3NH3PbI3 structures are well-formed. Structural Characterization. The dimensions, crystallinity, and morphology of the perovskite structures were examined in detail by transmission electron microscopy (TEM) (Figure 2). The bright-field TEM (BFTEM) micrograph obtained from P1 (Figure 2a) shows the formation of a well-faceted, elongated crystal with an average diameter of 400 nm and length 1000 nm,
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corroborating SEM analysis. Selected-area electron diffraction (SAED) shows a spot pattern (Figure 2b), evidencing the single-crystalline nature of the perovskite structure; the pattern could be indexed to the tetragonal phase of CH3NH3PbI3, in agreement with the literature.23 Highresolution TEM (HRTEM) (Figure 2c) reveals lattice fringes with d spacing of 6.0 Å, indexable to the (002) planes of the tetragonal phase of CH3NH3PbI3, consistent with the fast Fourier transform (FFT) pattern in the inset.
Figure 2. TEM analysis of perovskites; BFTEM, SAED patterns and HRTEM of P1 (a-c), P2 (d-f), and P3 (g-i) samples (inset: FFT patterns obtained from the corresponding HRTEM). It is to be noted that CH3NH3PbI3 formation takes place along the [001] direction which leads to the formation of elongated, well-faceted structures. The formation of elongated structures is not trivial, as tetragonal phase might exhibit shape anisotropy. Moreover, the presence of wellformed crystals of CH3NH3PbI3, as evident from SAED and HRTEM images, indicates that
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growth takes place under near-equilibrium conditions. In other words, oriented growth occurs through the attachment of nuclei along a particular crystallographic orientation. Such a phenomenon of oriented attachment is well established in entirely inorganic nanostructures grown from solutions under optimized conditions.24 Therefore, the growth of CH3NH3PbI3 structures can be explained by invoking the phenomena of oriented attachment, as amply supported by electron microscopy. We also conclude that under optimized conditions, sequential deposition method offers a rational control over the growth of such perovskite crystals. Likewise sample P2 was examined by TEM (Figure 2d-f). BFTEM micrographs (Figure 2d) show the presence of asymmetric structures along with elongated rods (inset; Figure 2d). The average dimension of asymmetric structures is ~400 nm and that of the elongated rod was estimated to be ~600 nm along the extension (Figure 2d; inset). The SAED displays spot pattern (Figure 2e) evidences that the CH3NH3PbI3 asymmetric structures are single-crystalline. This was further confirmed by the presence of clear lattice fringes in the corresponding HRTEM image of the structure (Figure 2f). The BFTEM image of P3 reveals the presence of aggregated structures (Figure 2g); such morphology could result from the coalescence of structures during the growth that is expected while using a single-step method for the deposition of perovskites. The SAED pattern and HRTEM image show that perovskite structures are crystalline in nature (Figure 2h, i).
Elemental Analysis. Although electron microscopy provided ample evidence for the formation of CH3NH3PbI3 structures under different conditions, it is well known that composition strongly dictates the properties of perovskites.9,13 Specifically, the pronounced diffusion length in perovskite-based devices has been ascribed to the presence of chloride ions.17 Therefore,
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examination of composition and atomic distribution becomes imperative and was carried out by performing EDS mapping in the STEM mode on all the three samples.
Figure 3. STEM-EDS analysis of individual CH3NH3PbI3 crystal; (a) STEM bright-field image, (b) STEM-EDS spectrum, (c) Pb map, (d) I map, and (e) N map. Figure 3a shows the bright-field STEM image of a well-faceted perovskite crystal. It is to be noted that SAED confirmed that it is a single crystal. Energy-dispersive X-ray spectroscopy in the STEM mode (STEM-EDS, Figure 3b) shows that Pb, I, and N are present in a perovskite single crystal. However, to within the detection limit of EDS (~1000 ppm) no feature corresponding to Cl was detected. It is to be noted that the reaction mixture contained 5 mol % of MACl. Colella et al.25 showed that in iodide-based perovskites; less than 3-4% of chloride content present in the reaction mixture could incorporate into the lattice. Thus, the concentration of chloride ions used here i.e., 5% mol MACl, would have easily led to their detection in our sample, if present. Moreover, Colella et al. documented that the considerable difference in the
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ionic radii limits the formation of a continuous solid solution.25 Furthermore, “STEM area mapping” of a single elongated perovskite crystal (Figure. 3c-e) reveals homogeneous distribution of the constituent atoms, i.e., Pb, I, and N, throughout the dimension of perovskite crystal. As we could not surprisingly, detect any signal corresponding to chlorine, the case for chloride ions possibly acting as growth directing agents becomes stronger. Further we analyzed P2 samples using STEM. Figure 4a shows the bright-field STEM image of asymmetric perovskite crystals. In addition to Pb, I, and N, STEM-EDS (see S.I.) reveals the presence of Cl in the sample. To examine the distribution of constituent elements further, “STEM area mapping” was carried out. Such mapping provides direct evidence of distribution of Pb throughout the CH3NH3PbI3 crystal (Figure. 4c). However, the distribution of Cl is found to be inhomogeneous (Figure 4f). The question that therefore arises is whether the Cl is present in the form of unconverted PbCl2 or in the mixed perovskite, i.e., CH3NH3PbClxI3-x.
Figure 4. STEM-EDS analysis of CH3NH3PbI3 asymmetric crystals; (a) STEM bright field image (b) STEM-EDS spot spectrum (c) Pb map, (d) N map, (e) I map, and (f) Cl map.
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To probe the source of Cl, we analyzed a single elongated perovskite rod using STEM-EDS. As evident from SEM analysis, these rods were grown along with the chloride-containing asymmetric structures (Figure 1a-b). Expectedly, STEM-EDS (see S.I.) confirmed the absence of Cl in the rods and area mapping (see S.I.) reveals a homogeneous distribution of the constituent atoms, i.e., Pb, I, and N throughout the perovskite rod (see S.I.). Furthermore, spot EDS spectrum under the STEM mode (Figure 4b) obtained from chloride containing region shows strong Cl signal but no feature corresponding to N was observed. Therefore, the chloride content in the asymmetric perovskite structures (Figure 4a) could be attributed to the presence of PbCl2, which apparently remained unconverted underneath the perovskite structure. Although CH3NH3PbI3 was obtained using PbCl2, i.e., in the presence of a high concentration of Cl, it appears that, under given conditions, the probability of incorporation of Cl in the final CH3NH3PbI3 material is negligible.
Figure 5. STEM-EDS analysis of aggregated CH3NH3PbI3 structures; (a) STEM bright-field image, (b) STEM-EDS spectrum, (c) Pb map, (d) N map, and (e) I map.
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Using a single step approach, Tidhar et al.22 reported that the addition of PbCl2 affects the crystal formation kinetics without being a part of the final product except in trace amount. Our observation is in agreement with their study as we also found that PbCl2 in two-step methodology improves the surface coverage of perovskite without being part of it (detection limit of characterization technique ~1000 ppm). However, we noted that the rate of conversion into CH3NH3PbX3 was found to be slower while using PbCl2 in comparison to PbI2. Figure 5a shows the bright-field STEM image of aggregated perovskite (P3) structures, complementing the BFTEM analysis. The STEM-EDS spectrum (Figure 5b) brings out the presence of Pb, I, and N in P3. However, no peak corresponding to Cl was detected in the aggregated perovskite structures. Furthermore, STEM-EDS area mapping (Figure 5c-e) evidences a homogeneous distribution of constituent atoms, i.e., Pb, N, and I within the aggregated perovskite structures. Surprisingly, we could not detect any signal corresponding to chlorine; it is to be noted that the reaction mixture contained PbCl2: MAI in the molar ratio of 1:3. Understanding the role and confirming the presence of Cl in perovskites is of great scientific interest and the studies carried out in this direction have not been conclusive so for. Nanova et al.26 documented excellent studies based on analytical transmission electron microscopy for the visualization of the mesoporous absorber layer in a perovskite solar cell. Docampo et al.27 reported that addition of chloride in the immersion solution leads to the enhancement of shortcircuit current of solution-processed planar heterojunction solar cells. However, authors’ were unable to detect any feature corresponding to chloride using EDX and electron energy loss spectroscopy. Using depth-profile XPS, Yu at al.28 detected negligible amount of Cl content in the fully annealed perovskite films. Grancini et al.29 concluded that the Cl induces a preferred
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orientation in the crystalline grains and observed that the mesoporous metal oxide scaffold retains a negligible amount of chlorine. Furthermore on the basis of DFT calculations, as compared to bulk perovskite, preferential position of chloride was found to be the TiO2 interface.30 In the formation of organo-lead halide perovskite films, Williams et al.31 elucidated the structural role played by the chloride but to within the detection limit of XPS, the feature characteristic of Cl 2p core electrons was found to be absent. Although on the basis of some elemental analysis techniques, presence of negligible amount of chloride have been reported32 however, it becomes imperative to determine the nature of chloride present in the samples, i.e., whether Cl is present in the form of PbCl2 or mixed halide perovskite. We investigated the chemical composition of three different CH3NH3PbI3 perovskites materials fabricated using different sources of chloride ions and methods. To within the detection limits, STEM-EDS reveals clearly that the CH3NH3PbI3 perovskite formed are chloride-free. Even though EDS signal corresponding to Cl was detected in the samples prepared by the sequential method with pure PbCl2 as the lead precursor, careful examination shows that this is attributable to unconverted lead chloride. In summary, our distinctive study indicates that under given conditions the chloride ions influence the growth of the CH3NH3PbI3 significantly without occupying the lattice. Experimental Section Photoanode preparation. A commercial paste (Dyesol 18 NRT) was spin-coated onto a precleaned TCO glass substrate (NSG 10, Nippon sheet glass, Japan) containing a ~70 nm-thick TiO2 blocking layer, followed by a series of sintering steps (325 °C for 5 min with 15 min ramp time, 375 °C for 5 min with 5 min ramp time, 450 °C for 15 min with 5 min ramp time, and 500 °C for 15 min with 5 min ramp time), and the sintered films were used as the photoanode. Prior
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to the deposition of TiO2 blocking layer, the FTO substrate was cleaned with a detergent, rinsed with water and ethanol, and then treated with a UV/O3 cleaner for 15 min. Deposition of perovskite using sequential method (P1). A 1.25 M PbI2 solution was prepared in N,N-dimethylformamide (DMF) solvent by constant stirring at 70 °C. The TiO2 photoanode films were then coated with PbI2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for 30 min. After cooling to room temperature, the films were dipped in a solution containing a mixture of CH3NH3I:CH3NH3Cl (95:5) wt% in 2-propanol (8 mg mL-1) for 4 min at 60 °C, rinsed with 2-propanol for 2 s and annealed at 110 °C for 30 min. Deposition of perovskite using sequential method (P2). A 1.25 M PbCl2 solution in dimethyl sulfoxide (DMSO) was prepared by constant stirring at 70 °C. The TiO2 photoanode films were then coated with PbCl2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for 30 min. After cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2-propanol (8 mg mL-1) for 4 min, rinsed with 2-propanol for 2 s and dried at 110 °C for 30 min. Deposition of perovskite using single-step method (P3). One mole of PbCl2 (278.1 mg) and three moles of CH3NH3I (158.9 mg) were dissolved in 1 mL of dimethyl sulfoxide solvent. After heating at 70 °C for 30 min the reaction mixture was deposited onto TiO2 photoanode films by spin-coating at 1500 rpm for 30 s, and annealed at 110 °C for 30 min. Deposition of CH3NH3PbI3 using sequential method (P4). A 1.25 M PbI2 solution in N,Ndimethylformamide (DMF) was prepared by constant stirring at 70 °C. The TiO2 photoanode films were then coated with PbI2 by spin-coating at 3000 rpm for 30 s, and dried at 70 °C for 30 min. After cooling to room temperature, the films were dipped in a solution of CH3NH3I in 2propanol (8 mg mL-1) for 4 min, subsequently rinsed with 2-propanol for 2 s and annealed at 70 °C for 30 min. All the samples were prepared in a controlled atmosphere, with humidity < 2%.
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Materials characterization. The CH3NH3PbI3 structures were examined by Osiris field-emission transmission electron microscope operating at an accelerating voltage of 200 kV, equipped with an Oxford energy-dispersive X-ray (EDS) detector. The TEM specimens of CH3NH3PbI3 were prepared by slow evaporation of diluted solutions, obtained by dispersion of powders by sonication in toluene, and deposited on a formvar-coated holey carbon copper grid under dry conditions (humidity
